1. Field of the Invention
This invention relates to thermal reactors used in semiconductor wafer processing operations and, more particularly, to a mechanism for rotating and vertically displacing a susceptor and for assisting the removal of a wafer from the thermal reactor.
2. Background
Semiconductor wafers are usually processed in a thermal reactor. To accomplish this, the wafer is inserted into the reactor, by means of a robot arm, and placed to rest on a graphite susceptor. The wafer is then processed and, once the processing operation is completed, the robot arm removes the wafer from the susceptor and reactor.
The manner in which the wafer has been transferred to and from the susceptor in the past is illustrated in FIG. 1. This figure is a schematic cross-section through a portion of a semiconductor thermal reactor 10 with a wafer 12 shown to be resting on a susceptor 14. For purposes of clarity the lower portion of the thermal reactor 10, the susceptor drive mechanism and the dome-shaped upper quartz window are not shown.
To initiate the wafer transfer operation the susceptor 14 moves downwards, in a two-step process. During the first of these steps, the susceptor 14 together with a wafer support cradle 16 which is located below the susceptor, move down to a lower position represented by the cradle shown in broken lines 16'. Then, during the second of these steps and with the cradle 16 remaining stationary, the susceptor moves further down to a lower position 14' also represented in broken lines. As a result wafer support pins 18, which are located in holes 20 formed through the susceptor 14, move through the susceptor and support the wafer 12 at a position 12'. This position 12' is some distance above the susceptor's lower position 14' but still below the position of the wafer before the transfer operation was commenced. The space betweens the wafer 12 and the susceptor 14 in this lowered configuration is sufficient to allow a robot arm (not illustrated) to access the chamber and move into position below the wafer.
Thereafter, both the susceptor 14 and the cradle 16 are together moved further down until the wafer 12 is supported entirely by the robot arm and clear of the wafer support pins 18 whereupon the robot arm removes the wafer from the thermal reactor 10.
To ensure uniform heating and distribution of reactant gases across the surface of the wafer 12, the susceptor 14 is rotated during the processing operation. Unfortunately, certain problems are associated with the arrangement as illustrated in FIG. 1 and, particularly, with the rotation of the susceptor 14.
As is shown in this figure, the wafer support pins 18 are located within holes 20 formed through the body of the susceptor 14. This means that the cradle 16 must be rotated at exactly the same speed as the susceptor is. If not, the support pins 18 bear against the inner walls of the holes 20 and cause abrasion which, in turn, results in particles which can contaminate the processing environment within the thermal reactor 10. As it is well known that contaminates within a semiconductor processing reactor 10 can be very detrimental to the quality of a processed semiconductor wafer, it will be apparent that the contaminating particles produced by this abrasion is unacceptable in certain wafer processing operations.
The problem of contaminating particles is further compounded by the fact that the susceptor 14 is made of graphite and the cradle 16 typically of stainless steel or quartz. Because of these different materials of construction, different rates of thermal expansion exist for the susceptor 14 and the cradle 16. Given that the processing temperatures within the thermal reactor 10 can exceed 1,000.degree. C., these differences in expansion are not insignificant. It therefore becomes impossible to position the pins 18 accurately within the holes 20. As a result, at certain temperatures, the pins 18 bear strongly against the inner walls of the holes 20 which further contributes to the abrasion described above and thereby adds to the number of particles which are generated.
A further problem associated with the illustrated apparatus results from the fairly large tolerance between the diameter of the support pins 18 and the holes 20. This large tolerance is necessary to maintain the alignment between the pins 18 and the holes 20 and to reduce, as far as possible, the abrasion effects described above. However, as a result of these tolerances, a space exists between the pin 18 and the inside wall of the hole 20. Processing gases from within the thermal reactor 10 are therefore able to pass between the pin and the inner wall of the hole 20 and mark or "burn" the back side of the wafer 12. This burning of the wafer 12 is, under certain circumstances, unacceptable from a quality control point of view and is, therefore, highly undesirable.
From the above, therefore, it is apparent that the need exists for a mechanism which can rotate the susceptor and displace the wafer therefrom which results in the minimum amount of contamination within the thermal reactor and, at the same time, reduces the amount of burn on the back side of the wafer.